my $gtf_file=$ARGV[0];
open(FH,"<$gtf_file") or die;
my @in=<FH>;
chomp(@in);
close(FH);
my $length = 0;
foreach my $line(@in){
	next if $line !~ /^chr/;
	my @splitin = split "\t",$line;
	$length = $length + abs($splitin[4] - $splitin[3]);
}
print "total length of this gtf file is: $length\n";